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Dz T T T G CAPE

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Dz T T T G CAPE Definitions & 1. CAPE Computations A. SBCAPE (Surface Based CAPE) Convective Available Potential Energy (CAPE) represents the vertically integrated positive buoyancy of a parcel experiencing adiabatic ascent. From the Skew-T Log P Diagram and Sounding Analysis Remote Training Module, RTM-230 (NWSTC, 2000) CAPE is expressed mathematically as Z EL æTvp - Tve ö CAPE = g ç ÷dz, ò ç T ÷ ZLFC è ve ø where Tvp is the virtual temperature of the parcel and Tve is the virtual temperature of the environment, Zel is the height of the equilibrium level, ZLFC is the level of free convection, and g is gravity. The units for CAPE are expressed in J/kg. Alternate forms of the CAPE equation do not use virtual temperature, but use environmental and parcel temperatures in degrees Celsius. In AWIPS, CAPE is labeled as the "Positive Energy Above LFC" on the Skew-T program, and is calculated using potential temperature. The parcel potential temperature and sounding potential temperature are computed at each level between the LFC and the EL. When the parcel temperature is greater than the sounding temperature, the two values are then summed up. Only one LFC is computed; thus, double passes of the parcel trace across the environmental temperature trace are not accounted for (e.g., in midlevel inversions). In addition, if there are small positive areas below the LFC above the negative areas, then AWIPS skew-T will not compute these values. See the AWIPS validation web site for more details. (A graphical depiction of the positive and negative areas on a sounding resulting from a rising parcel originating at ground level is shown below in Figure 1.) Note that Parcel Characteristics shown on the AWIPS Skew- T program are derived in different ways depending on the time of the sounding. The parcel parameters of T and Td are computed one of two ways: 1) If the sounding is a morning sounding (06 UTC - 18 UTC), then the parcel temperature is the computed forecast max temp, and the parcel dewpoint is the lowest 50 mb mean buoyancy in the sounding. This is labeled in the display as a modified parcel, indicating the parcel has the potential to reach these characteristics during the day. 2) If the sounding is a 00 UTC sounding, then the surface temperature and buoyancy are used as the parcel temperature and dewpoint, respectively. This assumes that the atmosphere is pretty much mixed out. This forms the basis of all the parameters on the right side of the table display. Arrays of parcel temperatures, dewpoints, and pressures are computed containing these parcel characteristics. (Note: In BUFKIT, starting with version 4.02, there are many options for computing and displaying CAPE, based on selectable parcel lifting level, but once the parcel's lifting level is defined, the CAPE computations are similar to AWIPS; i.e., it sums up the positive energy area above LFC to the EL and does not include any negative areas). However, in BUFKIT, one can account for midlevel inversions and more than two positive/negative areas on the sounding by using the ECAPE manual parcel lifting options (See Section C-Most Unstable CAPE). Note: Neither AWIPS Skew-T or BUFKIT CAPE computations incorporate a virtual temperature correction to the parcel path and environmental temperature trace to account for the effect of moisture on air density (buoyancy). NSHARP does incorporate a Virtual Temperature Correction in their thermodynamic computations. See this AWIPS sounding validation web site for information. This effect will slightly increase the temperature of the parcel and the environment in moist low levels. Also, when lifting the parcel with a virtual temperature correction, it does not exactly follow the moist adiabatic lapse rate since the skew- T moist adiabats have typically been constructed using temperature and not virtual temperature. For more description on how AWIPS calculates CAPE (and other parameters) see the AWIPS validation web site. For information on the various lifting methods employed on the Interactive Skew-T Program in AWIPS, see the AWIPS User's Manual. Strengths CAPE integrates a substantial portion of the thermodynamic information contained in a sounding. It is proportional to energy available for a rising parcel. CAPE provides an estimate of maximum updraft strength (Wmax) in convective storms by the relationship: Wmax = 2CAPE CAPE is a fundamental indicator of potential intensity of deep, moist convection. Operationally, CAPE is more popular than indices such as Lifted Index or K Index which uses temperature and dew point from only a few mandatory levels in a sounding. Research has shown that low-level CAPE may have relevance to tornado production. More CAPE in the lowest levels above the ground suggests stronger potential for large low-level accelerations and enhanced low-level mesocyclone intensification. A recent study, Davies (2002) showed that stronger supercell tornadoes tend to have more low-level CAPE (0-3 km) than non-tornadic storms (see this graph from Davies, 2002). Sensitive to both magnitude of buoyancy and the depth of Weaknesses integration. In AWIPS, there is no way to quantify layered CAPE, for example, surface to 3 km CAPE. Surface based computations do not account for layers that are not well mixed, and may grossly underestimate buoyancy in situations where parcels are experiencing elevated ascent. As in all parcel theory indices, CAPE assumes no mixing with the surrounding environment, and ignores effects of freezing and water loading. If ambient temperature is used instead of virtual temperature to calculate CAPE, lower CAPE values will result. See this graphic showing two forecast soundings from the Eta and LAPS models from 1200 UTC 25 March 2002 in southern Oklahoma. Both forecast soundings depicted zero net CAPE (Surface-based, most unstable CAPE and mean layer CAPE), which could be misapplied because there is CAPE in the sounding. To compute it, you must manually select (lift) the parcel from above the stable layer (~ 780 mb). On the other hand, the Eta BUFKIT sounding from the same time and location, because the parcel lifting level is interactively selectable from the most unstable level, came up with an elevated CAPE value of 735 J/kg. (Note: Severe hail occurred in the vicinity of this sounding across portions of southern and southeastern Oklahoma.) SBCAPE will overestimate realized instability when soundings possess shallow moist layers. SBCAPE value alone does not account for effects of vertical distribution of CAPE. The estimates of maximum updraft strength (W ) based on max CAPE are usually twice as high as in observed updrafts because of water loading and mixing effects. In well-organized convective storms, vertical velocity in updrafts are much closer to W . max Supercells can have strong updrafts even when the static instability, as measured by CAPE, is modest (See McCaul and Weisman, 2001). This is due to vertical shear effects. The virtual temperature correction can increase low-level CAPE calculations by 20-50 J/kg (see this graph from Davies, 2002). Definitions & B. Mean layer CAPE (often abbreviated Computations MLCAPE) On SPC discussions, this is the CAPE calculated using the lowest 100 mb AGL mean layer temperature and moisture values (same as in BUFKIT). In the AWIPS interactive Skew-T, a similar computation is the "Mean Temp Lift" parcel lifting option. However this uses the lowest 50 mb mean potential temperature layer above the surface, so these values will differ slightly from BUFKIT. MLCAPE is more representative of realized buoyancy Strengths because it incorporates parcel mixing effects. MLCAPE and low-level lapse rates have been shown to be two good parameters for discrimination of general thunderstorms (i.e., whether convection produces lightning). See Craven et al., 2002. Soundings taken in proximity of thunderstorms usually possess more than 250 J/kg of MLCAPE. MLCAPE, when combined with LCL height, has been shown to be a very good discriminator for tornadic supercells (see Craven et al. 2002). Weaknesses MLCAPE is likely to underestimate elevated and/or surface- based buoyancy if layers are not well-mixed. MLCAPE has more difficulty in discriminating between general thunderstorms and severe thunderstorms (lots of overlap). See Craven et al. (2002). Definitions & C. Most Unstable CAPE (often Computations abbreviated MUCAPE) In SPC discussions, this is CAPE calculated by using a parcel from a pressure level which results in the most unstable CAPE value possible in the lowest 300 mb AGL. (Note: this is different than PMAX on AWIPS Skew-T, which uses the most unstable parcel in the lowest 50 mb, based on highest wet bulb temperature.) For information on the various lifting methods employed on the Interactive Skew-T Program in AWIPS, see the AWIPS User's Manual. The best way to assess elevated instability using AWIPS skew-T program is to visually inspect the sounding and pick the parcel level where the most CAPE results above the surface (note: use the "User Select" option for lifting method). Usually, this is between 900 and 700 mb. See this example of selecting a parcel on an AWIPS skew-T below. Strengths MUCAPE is the best sounding measure for elevated buoyancy and assessing potential for elevated convection. In BUFKIT, one can interactively compute CAPE from any level. See this BUFKIT sounding example of a parcel lifted from 764 mb and the resulting CAPE of 957 J/kg. Note the following differences in the characteristics of the air parcels used to compute CAPE for various model analysis packages: 1) ETA "surface" CAPE (from Volume Browser) uses the highest qe in the 70 mb just above the surface. (Note in BUFKIT, the average temperature and dew point in the lowest 100 mb is the default CAPE, but parcel lifting levels can be modified more easily than in AWIPS ).
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